In general, an adsorption/desorption type energy storage device includes two electrodes, a separator interposed between the two electrodes for electrically insulating them from each other, an electrolyte and an outer casing for housing the above components. While the conventional battery converts chemical energy into electric energy via redox reactions, the adsorption/desorption type energy storage device stores energy in it via surface adsorption of electric charges. Herein, adsorption of electric charges occurs at the interface between each electrode and the electrolyte, and thus the above energy storage device desirably shows a higher charge/discharge rate as compared to the conventional battery.
Generally, the most important material for forming an electrochemical device based on adsorption/desorption mechanism is an electrode. An electrode generally includes a current collector and an electrode active material adhered onto the current collector. Herein, the current collector and the electrode active material serve to perform electron conduction and electric charge adsorption from an electrolyte, respectively. As the electrode active material for such adsorption/desorption type electrochemical devices, activated carbon is the most widely used, so that many studies have been conducted to increase specific surface area of activated carbon. In addition to activated carbon, electrode active materials that have been used recently include RuO2, IrO2, MnO2 and conductive polymers conducting faradic reactions.
Meanwhile, the quality of an adsorption/desorption type electrochemical device depends on the capacity, voltage and internal resistance thereof. Methods for increasing discharge energy in an electrochemical device based on ion adsorption/desorption mechanism may be classified into the following three types of methods: a method for increasing a cell voltage, a method for reducing the internal resistance of a cell, and a method for developing a novel electrode active material. Among those, methods currently used for increasing a cell voltage in a system using an adsorption/desorption type electrochemical device include a method for connecting a plurality of cells in series. Additionally, many studies have been conducted to reduce the internal resistance of such an electrochemical device by improving physical properties of a binder that allows an electrode active material to be bound to a current collector. Further, many attempts have been made to develop novel electrode active materials. However, any method for imparting high capacity and high output to an electrochemical device while reducing the internal resistance has not been suggested yet.